The present invention relates generally to superconducting quantum interference devices (SQUIDs), and more particularly to an improved, low noise, radiofrequency amplifier using a dc SQUID as the input amplifying element.
Conventional radiofrequency amplifiers operating at room temperature commonly exhibit a substantial amount of internal electronic noise. Such noise, of course, limits the minimum level of input signal that can be detected by the amplifier. The radiofrequency amplifier described herein has a noise level two orders of magnitude below (i.e. 1/100th of) that achieved by a typical room-temperature amplifier operating at frequencies up to 100 MHz.
The radiofrequency amplifier described herein uses a supercooled dc SQUID as the input amplifying element. In order to explain the principle of operation, and to teach the invention, an understanding of a dc SQUID is necessary.
Many metals and alloys become superconducting when they are cooled to a few degrees absolute. It has been theorized that in the superconducting state, electrons with equal and opposite momenta bind together to form "Cooper pairs" with charge 2e. Each pair has zero net momentum and all pairs in a given superconductor can be described by a single macroscopic wave function with the same quantum-mechanical phase. One demonstration of the existence of the macroscopic wave function is flux-quantization. Suppose a ring is cooled through its superconducting transition temperature in the presence of an axial magnetic field and the field is then removed. A flux, .PHI., is trapped in the ring, maintained by persistent supercurrent carried without resistance by the Cooper pairs. However, the flux cannot take an arbitrary value, but is quantized in units of the flux quantum, EQU .PHI.=h/2e.perspectiveto.2.times.10.sup.-15 Wb,
where h is Planck's constant and e is the charge on an electron. Thus, .PHI.=n.PHI..sub.o, where n is an integer.
Josephson tunneling provides a second illustration of the macroscopic quantum state. In 1962, Brian Josephson proposed that Cooper pairs could tunnel through a thin insulating barrier separating two superconductors. Since the tunneling involves electron pairs, rather then single electrons, the current flows through this junction as a supercurrent, and no voltage appears between the two superconductors, i.e. across the insulating barrier. This is the dc Josephson effect. The supercurrent, I, develops a phase difference, .phi.=.phi..sub.1 -.phi..sub.2 =between the two superconductors according to the current-phase relation I=I.sub.o sin.phi..
The critical current, I.sub.o, the maximum supercurrent the junction can sustain, depends on temperature and the properties of the barrier. If a current greater than I.sub.o is forced through the junction, a voltage, V, will appear across the barrier, and part of the current will flow dissipatively. The Josephson current will persist, but will now oscillate in time at a frequency, .nu., wherein EQU .nu.=d.phi./2.pi.dt=2eV/h=V/.PHI..sub.o.
This is the ac Josephson effect. As the current is increased from zero, a voltage jump occurs at I=I.sub.o ; when the current is reduced, the voltage does not return to zero until the current is almost zero. The hysteresis can be removed by a resistive "shunt"--a strip of normal metal connecting the two superconductors. Part of the current at low voltages in a shunted junction is carried by the ac supercurrent, which has a non-zero time average.
The Superconducting Quantum Interference Device (SQUID) neatly combines flux quantization and Josephson tunneling. SQUIDs come in two varieties, dc and rf, and are by far the most highly developed and widely used Josephson devices.
The dc SQUID consists of two shunted Josephson junctions interrupting a superconducting ring. The constant bias current, I.sub.B, (greater than 2I.sub.o) maintains a non-zero voltage across the SQUID, which has a non-hysteretic current-voltage characteristic. If the magnetic flux, .PHI., threading the SQUID ring, is slowly varied, the critical current will oscillate as a function of .PHI. with a period that is just .PHI..sub.o. The critical current is a maximum for .PHI.=n.PHI..sub.o, and a minimum for .PHI.=(n+1/2).PHI..sub.o. The effect of the magnetic field is to change the phase differences between the two junctions. The oscillating behavior arises from interference between the wave functions at the two junctions in a manner analogous to interference in optics--hence the term "Interference Device." At low voltages, the current-voltage characteristic is also modulated because the current contains a contribution from the time-averaged ac supercurrent. As a result, when the SQUID is biased with a constant current, the voltage is periodic in .PHI. with period .PHI..sub.o.
For several years, dc SQUIDs have been used to measure extremely small values of voltage, magnetic flux and magnetic flux gradients at low frequencies.
An rf SQUID consists of a single, non-hysteretic Josephson junction interrupting a superconducting ring. The rf SQUID is operated by applying a radiofrequency current, at typically 20 or 30 MHz, to an LC-resonant circuit, the inductance of which is coupled to the SQUID. The radiofrequency current thus induces a radiofrequency flux into the SQUID. When the quasistatic flux in the SQUID is changed, the amplitude across the resonant circuit oscillates, again with a period .phi..sub.o. The voltage across the resonant circuit is then amplified and detected. An rf SQUID amplifier is restricted to signal frequencies much less than the radiofrequency pump frequency.